Enumerating the circular buffer does not produce anything

After adding some random items to the circular buffer, using foreach or any function to enumerate the buffer does not produce anything

Here is a sample code:

`int[] values = new int[20];
Random rnd = new Random();

for (int i = 0; i < values.Length; i++)
values[i] = rnd.Next(1, 99);

CircularBuffer buffer = new CircularBuffer(values.Length / 2);

Console.WriteLine(string.Join(", ", values));

foreach (int value in values)
buffer.Add(value);

Console.WriteLine(string.Join(", ", buffer));

Console.ReadKey();`

**Expected behavior**
2 lines should be printed: 1 for the array content joined by ',' and another line for the content of the buffer joined by ','

查看 CircularBuffer 類別，它可能位於 DataStructures 資料夾中。問題在於新增項目後列舉 (foreach) 無法正常運作。檢查 GetEnumerator 方法，以及緩衝區如何追蹤項目。使用提供的範例程式碼撰寫測試以重現該問題，然後修正列舉邏輯。

由索引模型根據 Issue 內容生成。